Mobile Bay is an inlet of the Gulf of Mexico, lying within the state of Alabama in the United States. Its mouth is formed by the Fort Morgan Peninsula on the eastern side and Dauphin Island, a barrier island on the western side. The Mobile River and Tensaw River empty into the northern end of the bay, making it an estuary. Several smaller rivers also empty into the bay: Dog River, Deer River, and Fowl River on the western side of the bay, and Fish River on the eastern side. Mobile Bay is the fourth largest estuary in the United States with a discharge of 62,000 cubic feet (1,800 m3) of water per second. Annually, and often several times during the summer months, the fish and crustaceans will swarm the shallow coastline and shore of the bay. This event, appropriately named a jubilee, draws a large crowd because of the abundance of fresh, easily caught seafood it yields. Mobile Bay is the only place on earth where jubilees are a common occurrence.
Mobile Bay is 413 square miles (1,070 km2) in area. It is 31 miles (50 km) long by a maximum width of 24 miles (39 km). The deepest areas of the bay are located within the shipping channel, sometimes in excess of 75 feet (23 m) deep, but the average depth of the bay is 10 feet (3 m).
Spanish explorers were sailing into the area of Mobile Bay as early as 1500, with the bay being marked on early maps as the Bahía del Espíritu Santo (Bay of the Holy Spirit). The area was explored in more detail in 1516 by Diego de Miruelo and in 1519 by Alonso Álvarez de Pineda. In 1528, Pánfilo de Narváez travelled through what was likely the Mobile Bay area, encountering Native Americans who fled and burned their towns at the approach of the expedition. This response was a prelude to the journeys of Hernando de Soto, more than eleven years later.